The development of phage display technology, whereby non-native (heterologous) polypeptides or proteins are expressed and anchored on the surface (“displayed”) of a bacteriophage, is a powerful tool for identifying molecules possessing biological activities of interest, for example, peptide ligands that bind with high specificity and/or affinity to a given target molecule. Libraries of synthetic oligonucleotides can be cloned in frame into the coding sequence of genes encoding a phage surface protein, for example gene III or gene VIII of phage M13. These clones, when expressed, are “displayed” on the phage surface as a plurality, due to the variation in sequence of the oligonucleotides used, of peptide-capsid fusion proteins. These peptide display libraries are then screened for binding to target molecules, usually by affinity selection or “biopanning” (Ladner, R. et al., 1993; Kay et al., 1996; Hoogenboom, H. et al., 1997).
Phage display library screening is highly advantageous over other screening methods due to the vast number of different polypeptides (typically exceeding 1×109) that can be contained in a single phage display library. This allows for the screening of a highly diverse library in a single screening step. Display of small peptides or single chain proteins on phage is advantageous as long as intracellular processing or post-translational modification (of which phage or prokaryotic hosts are not capable) are not necessary or desired. For example, effective display of a heterologous polypeptide may require various post-translational modifications, intracellular structures, and a compliment of specialized enzymes and chaperone proteins that are necessary to transport, to glycosylate, to conform, to assemble, and to anchor the display polypeptide properly on the surface of the host cell; however, none of these processes can be accomplished by bacteriophage or prokaryotic cell processes.
For the display of more complex eukaryotic proteins, for example multi-chain polypeptides including immunoglobulins and functional fragments thereof (e.g., Fabs), or the extracellular domains of MHC molecules or T cell receptor molecules, there are additional problems to overcome: coordinated expression of the component chains at the levels of expression sufficient to produce multi-chain products, transport and secretion of each chain while still accomplishing association into a functional multi-chain polypeptide, and immobilization (anchoring) of at least one chain of the multi-chain polypeptide at the host cell surface (i.e., for display), while retaining the proper assembly and functionality outside the host cell of the multi-chain polypeptide product.
Display systems utilizing eukaryotic cells, such as yeast, have been reported for expressing and displaying single chain polypeptides (Boder, E. and Wittrup, K., 1998; Horwitz, A. et al., 1988; Kieke, M. et al., 1997; Kieke, M. et al., 1999; WO 94/18330; WO 99/36569), however the need exists for improved eukaryotic systems for the expression and functional display of multi-chain polypeptides, particularly immunoglobulins and fragments thereof. Moreover, there is a need in the art for polypeptide display in a system that harnesses the power of phage display and the processing advantages of eukaryotic host cells. For example, in contrast to phage display libraries, the maximum practical size, or “diversity”, of a library that can be expressed in and displayed on the surface of a eukaryotic host cell is about 106 to 107.
These and other technical problems have obstructed the advance of biological tools and techniques useful for identifying novel molecules, which possess biological activities of interest. Because of these technical problems, there has been no report to date of materials or methods for the successful construction of a multi-chain eukaryotic display vector, of the successful display of a multi-chain polypeptide (such as an antibody or a Fab fragment) on the surface of a eukaryotic host cell (such as yeast), of the creation of a multi-chain polypeptide display library in eukaryotic host cells, or of the successful use of such libraries to detect and to isolate specific multi-chain polypeptides of interest (for example, on the basis of binding specificity or affinity for a target molecule).